Can I create a giveaway with an out of stock key?

No, you shouldn't. The winner needs to be able to activate the game in their account within 7 days of the giveaway's conclusion, if you give them a gift link to an out of stock key you aren't really providing them with a working copy of the game, or at least not within the expected time.

I had to wait almost 3 months to mark the giveaway for "Milky Way Prince – The Vampire Star" as received. On hindsight, I should just have marked the giveaway as "not received" because I didn't know when the keys from humble would be restocked. My advice is to wait for the keys to be restocked before you create a giveaway

You can do a gift link but you might get hit with a not received for a long time, better to wait for it being in stock

You could make a giveaway in a group that allows giveaway deletion if key is no longer working?
https://steamcommunity.com/groups/dgikinw - This Group is kind of Popular